Dynamics of an enzyme polymorphism in the isopod, Sphaeroma rugicauda (Leach). II. Sexual, gametic and fecundity selection

Abstract

The estuarine isopod Sphaeroma rugicauda (Leach) exhibits a diallelic polymorphism at the locus coding for the enzyme Pgi. An investigation carried out during the animal’s breeding season (June–July) in 1979 and 1980 indicated that the animals were mating at random. It also showed the s/s-homozygote females to be the earliest to release offspring and to be the most fecund of the three genotypes (f/f, f/s and s/s). Further, data from the wild population in 1980 and laboratory experiments indicated that male s-gametes were selected against in s/s females, resulting in offspring ratio distortions in f/s (male) × s/s (female) crosses, in favour of heterozygotes. The net effect of these selective pressures did not, however, significantly alter the genotype frequencies (found in the offspring) from expected values. © 1983, The Genetical Society of Great Britain.